When selecting foods, a consumer typically considers its appearance, flavor, texture and nutritional value. Of these four attributes, it is often only the appearance that can be employed for the selection of the food product. The appearance is particularly impacted by the observed color. For many foods the favorable color is typically that from naturally occurring pigments such as chlorophylls, carotenoids and anthocyanins in a food as displayed in its mature state freshly removed from the environment that it naturally matures, such as freshly picked ripe fruit or freshly caught seafood. With the progression of time, the color can change by the inclusion or substitution of pigments resulting from both enzymatic and non-enzymatic reactions. Enzymatic browning is one of the most important color reactions that affect fruits, vegetables and seafood. This browning is catalyzed by the enzyme polyphenol oxidase (PPO) (1,2 benzenediol; oxygen oxidoreductase, EC1.10.3.1), which is a copper-containing enzyme that catalyzes the oxidation of o-diphenols to o-quinones. PPO is also referred to as phenoloxidase, phenolase, monophenol oxidase, diphenol oxidase and tyrosinase. Browning not only affects color, but can also adversely affect flavor and nutritional value.
Projected increases in fruit and vegetable markets require that enzymatic browning be understood and, more importantly, controlled. Enzymatic browning is devastating to the distribution of many exotic fruits and vegetables, particularly that of tropical and subtropical products. It has been estimated that more than 50 percent of losses during fruit distribution occur because of enzymatic browning. In addition to tropical and subtropical fruits and vegetables, products as diverse as lettuce, potatoes, sweet potato, breadfruit, yam, mushrooms, apples, avocados, bananas, grapes, and peaches are susceptible to significant losses during distribution due to browning. The closer to purchase by the consumer that browning occurs, the greater the economic losses incurred due to the storage and handling costs prior to this point in the distribution process. Therefore, controlling browning from harvest to consumer is critical for the maintenance of economic value to the agriculturist and food processor.
Polyphenol oxidases are believed to be important to the prevention of insects and microorganisms from attacking plants and are involved in the wound response of plants and plant products to insects, microorganisms and bruising. A fruit or vegetable's susceptibility to disease and infestation increases as it ripens because of a decline in its phenolic content. Phenoloxidase enzymes, endogenous to fruits and vegetables, catalyze production of quinones from phenolic constituents. These quinones subsequently undergo polymerization reactions that produce melanins, which exhibit both antibacterial and antifungal activity and assist in keeping the fruit and/or vegetable physiologically wholesome. Research on the antibacterial, anticancer and antioxidant nature of melanins has triggered considerable interest in enzymatic browning and has led to nutritional recommendations for increased consumption of fruits and vegetables. Convenience forms of these foods are particularly susceptible to enzymatic browning. Enzymatic browning does not occur in intact plant cells since phenolic compounds in cell vacuoles are separated from the polyphenol oxidase in the cytoplasm. Upon slicing, cutting, grating, pulping, or juicing, brown pigments form. The organoleptic and biochemical characteristics of fruits and vegetables are altered by pigment formation. The rate of enzymatic browning in fruit and vegetables is governed by the active polyphenol oxidase content of the tissues, the phenolic content of the tissue, pH, temperature and oxygen availability within the tissue. Attempt to control enzymatic browning have focused of the elimination of one or more of these governing factors.
Temperature control of browning is carried out by heating, blanching, or cooling, by refrigeration or freezing. Blanching is nutritionally disadvantageous, resulting in losses in vitamins, flavors, colors, texture, carbohydrates and other water-soluble components. Blanching is also technically disadvantageous as it requires large amounts of water and energy and typically has waste disposal costs. Refrigeration adds costs throughout the distribution and retailing process, but is commonly employed for the prevention of browning in fruit, vegetables, and seafood. Freezing causes changes in texture and other freshness characteristics and cm also lead to decompartmentalization of certain enzymes, substrates, and/or activators by cell disruption facilitating enzyme activity upon thawing of the food.
Other treatments include dehydration, irradiation, high pressure treatment, super critical CO2 treatment, ultrafiltration and ultracentrifugation. Of these methods, dehydration affects the texture and flavor of food, irradiation requires high levels of radiation to denature the polyphenol oxidase enzyme, super critical CO2 requires processing at pressures in excess of 50 atmospheres yet polyphenol oxidase is highly pressure resistant, and ultrafiltration and ultracentrifugation are processes that are effective only for liquids that effect the nutritional value and require significant processing costs. For these reasons such methods do not provide a general cost effective method to control enzymatic browning.
Enzymatic browning has been addressed primarily by the use of chemical inhibiting agents. Such inhibitors can target the enzyme, the substrates (oxygen and polyphenols) or the brown products of the reaction. Inhibitors that act directly on polyphenol oxidase are often classified as members of two groups. The first group consists of metal ion chelators, and includes azide, cyanide, carbon monoxide, halide ions and tropolone. The second group of inhibitors consists of aromatic carboxylic acids of the benzoic and cinnamic series which behave as competitive inhibitors of polyphenol oxidase by their structural similarity with phenolic substrates.
Substrate inhibitors remove either the oxygen or the phenolic substrate. The removal of oxygen can result in the promotion of anaerobic metabolic reactions in the food that can lead to breakdown with adverse effects on the flavor of the foods. Specific adsorbents can be used to remove phenolic compounds from foods. For example, cyclodextrins have been use for the removal of phenolic compounds from raw fruit and vegetable juices. Enzymatic modification of phenolic substrates has been examined for inhibition of polyphenol oxidase activity, however the cost of these enzymes is considered prohibitive towards the commercial development of this method. The products of diphenol oxidation, O-quinones, form dimers of the original phenol, which subsequently oxidize to form oligomers with varying color intensities. Ascorbic acid, thiol compounds, sulphites, and amino acids have displayed the capability of inhibiting dimer formation and oxidation, by reducing O-quinones to O-diphenols, or by formation of colorless addition products.
The use of browning inhibitors in food is restricted by considerations relevant to toxicity, wholesomeness, and their effect on taste, texture, and cost. Browning inhibitors have been classified by their primary mode of action as: (1) reducing agents; (2) acidulants; (3) chelating agents; (4) complexing agents; (5) enzyme inhibitors; and (6) enzyme treatments. Sulphites, considered reducing agents, are the most widely used inhibitors of enzymatic browning. Sulfites are subject to regulatory restrictions because of potential adverse health effects. Many reports have described allergic reactions in humans following the ingestion of sulphite-treated foods, frequently by hypersensitive asthmatics. Sulphites levels in food processing are based on their theoretical yields of sulfur dioxide. The Joint Expert Committee on Food Additives (JECFA) of the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) recommend sulphite daily intake be limited to less than 0.7 mg sulphur dioxide per kg of body weight, and significant effort is being made to identify appropriate substitutes. Other widely used inhibitors include: reducing agents such as ascorbic acid, erythorbic acid, cysteine, synthetic antioxidants (such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutyl hydroxyquinone (TBHQ) and propyl gallate (PG)), and plant based phenolic compounds (such as tocopherols, flavonoid compounds, cinnamic acid derivatives, and coumarins); acidulants such as citric, malic, and phosphoric acids; chelators such as sorbic acid, polycarboxylic acids (citric, malic, tartaric, oxalic, and succinic acids), polyphosphates (ATP and pyrophosphates), macromolecules (porphyrins, proteins), and EDTA; and enzyme inhibitors such as aromatic carboxylic acids, substituted resorcinols, halide salts, honey, amino acids, and proteins.
The search for effective affordable browning inhibitors continues. Inhibitors that are beneficial or, at least, non-toxic, and can be used at sufficient levels without adversely affecting the sensory characteristic of the foods, yet are cost effective, remain a need in the evolving food industry.